Semiconductor infrared detectors sense electromagnetic radiation, typically, in one of two spectral bands (wavelengths between 3 to 5 .mu.m and 8 to 13 .mu.m). Semiconductor infrared detectors have been incorporated into integrated circuits and used for a wide variety of purposes, including the acquisition and tracking of targets, and the detection and measurement of gases.
Two color infrared detectors have been fabricated, which allow simultaneous detection of infrared radiation at two distinct wavelengths. The simultaneous detection of two wavelengths of infrared radiation allows a user to more accurately calculate target locations or measure gas concentrations. In that regard, two color infrared detectors have been very useful in military, environmental monitoring, medical diagnostics, and industrial control process applications. An exemplary two color infrared detector is described in commonly-assigned U.S. Pat. No. 5,300,777 to Goodwin, which is incorporated herein by reference.
However, notwithstanding the advantages of two color infrared detectors over one color detectors, the two color infrared detectors are relatively difficult and expensive to manufacture.
For example, one type of a two color infrared detector is a triple-layer-heterojunction diode. This device consists of two infrared sensitive N-type semiconductor layers separated by a P-type semiconductor layer (i.e., two back-to-back diodes). The P-type layer is insensitive to infrared radiation. Each N-type layer is tuned to a separate infrared wavelength. Unfortunately, this structure requires the additional P-type layer and its related interconnections, and two separate sensing circuits to sense the two wavelengths. Furthermore, the existing photodiodes have problems collecting infrared radiation at the higher cutoff wavelengths without the use of additional cryogenic cooling, which increases the detectors' costs and overall system complexity.
A second device type of a two color infrared detector uses a heterojunction diode in combination with a metal insulator semiconductor (MIS) device. This type of two color infrared detector is described in the Goodwin Patent cited above. However, MIS devices (e.g., MIS capacitors) generate relatively high levels of dark currents (currents generated by defects in the crystal lattice) and also require extensive cooling.
Two color semiconductor infrared devices are typically manufactured in an N.times.M array having twice the usual number of individual detector elements. Half of the elements are tuned to one infrared wavelength, and the other half are tuned to the second infrared wavelength. In the two types of devices described above, the complexity of the circuit element arrays significantly increases the fabrication cost for the devices, and contributes to the generation of dark currents.
Accordingly, a need has arisen for a semiconductor infrared detector that can detect two bandwidths of infrared radiation simultaneously, operate at higher temperatures without extensive cooling, provide a relatively high immunity to the generation of dark currents, and minimize the cost and complexity of the integrated circuit.